1. Field of the Invention
The present invention relates generally to current collectors used in electrochemical cells. More particularly, the present invention relates in one embodiment to a current collector that is integrally provided with a solid area that enables the direct measurement of the thickness of a coating on the current collector.
2. Description of Related Art
Present electrochemical cell designs utilize two primary construction methods. Either the internal electrodes are spirally wound, or they are assembled in a multiple plate configuration. In either case, each of the positive and negative electrodes is comprised of a current collector and active chemical constituents contacted thereto. The current collector can be a conductive foil or screen.
For example, U.S. Pat. No. 5,312,458 to Muffoletto et al., which is assigned to the assignee of the present invention and incorporated herein by reference, describes the construction of one electrochemical cell having a current collector comprising wing-like portions which are folded into electrical association with a central electrode of an opposite polarity.
U.S. Pat. No. 6,893,777 to Probst, which is also assigned to the assignee of the present invention and incorporated herein by reference, describes the construction of another electrochemical cell comprised of a current collector having non-symmetric grid pattern converging at a common focal point.
The current collectors of these electrochemical cells are typically formed from thin screens of a conductive material such as nickel, aluminum, copper, stainless steel, tantalum, cobalt, and titanium, and alloys thereof. Prior to incorporating such a current collector screen into an electrochemical cell, it is known and preferable to coat the screen before contacting the active material thereto. Carbonaceous materials are suitable for this purpose. For example, U.S. Pat. No. 6,451,483 to Probst et al., which is assigned to the assignee of the present invention and incorporated herein by reference, describes a prior art Li/CFx cell including a titanium cathode current collector screen coated with a thin layer of graphite/carbon paint. In addition to increasing the electrical conductivity between the Li/CFx active material and the current collector, the graphite/carbon paint serves to prevent direct contact at the interface between the current collector and the active material.
One preferred formulation of such a graphite/carbon paint is described in U.S. Pat. No. 6,767,670 to Paulot et al., which is assigned to the assignee of the present invention and incorporated herein by reference. In this United States patent, at column 3, line 52, it is disclosed that, “The preferred current collector material is titanium, and most preferably the titanium cathode current collector has a thin layer of a carbonaceous material applied thereto. The coating is provided in a range of about of about 0.0001 inches to about 0.0010 inches, and more preferably in a range of about 0.0004 to 0.0005 inches (10 microns to about 12.7 microns).”
It is further disclosed at column 3, line 66 through column 4, line 16, “According to the present invention, a finely divided graphite pigment in an alcohol-based epoxy resin solution is used as the coating material. One of dipping, painting, doctor-blading, pressurized air atomization spraying, aerosolized spraying, or sol-gel deposition is used to contact the carbonaceous material to the current collector substrate. Spraying is a preferred method.”
“A particularly preferred material is commercially available from Acheson Industries, Inc., Port Huron, Mich. under the designation ELECTRODAG 213. This material is a colloidal suspension of graphite, propylene glycol, methyl ether acetate, toluene, formaldehyde, xylene, 2-butoxyethanol and proprietary epoxy and thermoset resins. The thusly-coated substrate is then sintered at a temperature of about 230° C. to about 350° C. for about 30 minutes to 1.5 hours. More preferably, the carbonaceous coating is applied to a thickness of about 0.0004 inches and sintered at about 300° C. for at least about one hour.”
For this exemplary carbonaceous coating and for other suitable carbonaceous coatings applied to the surface of a current collector, one must ensure that the coating is applied in a sufficient thickness to protect the current collector from corrosion and other deleterious effects, yet not be so thick as to occupy significant internal volume in the cell. Such internal volume is better used for active electrochemical materials, in order to maximize the total capacity and discharge rates of the cell.
There is, therefore, a need in the manufacturing of such current collectors to control the thickness of the carbonaceous coating applied thereto. A prerequisite to such coating thickness control is an accurate measurement of the coating thickness deposited on the surface of the current collector. Such a measurement may be made on a sample of current collectors taken during the manufacturing process, but it is preferable that measurements may be made on each individual current collector screen that is produced. In the latter case, it is obviously required that the measurement method be non-destructive with respect to the coating and the collector screen. It is also preferable that the capability be provided to make coating thickness measurements at multiple locations on a collector screen surface.
One preferred known non-destructive measurement method for measuring the thickness of coatings on substrates is the “Beta Backscatter” method. Beta rays are electrons emitted from unstable radioisotopes. In this method, a highly collimated beta ray source is directed at a coated sample such as a carbonaceous-coated titanium substrate as provided in the present invention. The electrons penetrate the coating material and are reflected back (back scattered) toward the beta ray source. The back-reflected electrons are collected and counted with a Geiger-Mueller tube; the resulting electron count is related to the thickness of the coating, and such thickness is calculated according to known algorithms. For further information on Beta Backscatter techniques for coating thickness measurement, reference may be had to the publication, “Standard Test Method for Measurement of Coating Thickness By the Beta Backscatter Method,” Designation: B 567-98 (Reapproved 2003), of ASTM International Inc. of West Conshohocken, Pa. (ASTM International Inc. was formerly known as the American Society for Testing and Materials).
As is disclosed at page 3, paragraph 7.2 of this publication, one requirement for obtaining accurate measurements of coating thickness using the beta backscatter method is that the atomic number of the coating material must be sufficiently different (by at least five atomic numbers) from the atomic number of the substrate material upon which it is coated. Such is clearly the case for the carbonaceous coatings applied to the metallic conductive collector screens of the present invention, used in the aforementioned electrochemical cells.
As is disclosed at page 3, paragraph 7.2.2 of this publication, a second requirement is that, “it is essential that the aperture be smaller than the coated area of the surface on which the measurement is made.” In this context, the “aperture” is the aperture of the beta backscatter instrument defined on page 1, paragraph 3.1.2 as “the opening of the mask abutting the test specimen.” In other words, within the perimeter of the mask opening is the coated area for which the thickness is measured. This area within the mask opening must be a continuous solid area of coating and substrate, free of any through holes or other gross surface irregularities.
This requirement for aperture size presents a problem in making measurements of prior art current collector screens by the beta backscatter technique. Due to the geometry and size of the mesh patterns of prior art collector screens, there is not provided in these collector screens a large enough section of solid area that is larger than the size of the apertures of typical beta backscatter thickness measurement instruments.